Please support MSYS2 MINGW64 #5

The project is obviously designed for MSVC as many of the submodules are indeed already available as packages on MSYS2 MINGW64. Some submodules like assimp doesn't build out of the box on MSYS2 MINGW64, the MSYS2 MINGW64 developers had to patch it to make the mingw-w64-x86_64-assimp package. It's better to check if the dependencies are available or not before falling back to build from source using the submodules.

Update: the problems with assimp seems to be because the submodule is too old, not because assimp needs to be patched to build on MSYS2 MINGW64 though. Indeed all of the submodules are too old.